Frequency selective filters are ubiquitous in communication devices, such as mobile telephones, wireless Internet devices, etc. Among other technologies, bulk acoustic wave (BAW) devices, surface acoustic wave (SAW) devices, thin film bulk acoustic resonator (FBAR) devices and coupled resonator filters (CRF) may be employed as filters in appropriate devices.
FBARs are similar in principle to bulk acoustic resonators such as quartz, but are scaled down to resonate at GHz frequencies. A typical implementation of an acoustic resonator comprises a layer of piezoelectric material arranged between two metal electrodes. Common piezoelectric materials include, for example, aluminum nitride (AlN) and zinc oxide (ZnO).
FIG. 1 shows an exemplary resonator 10 which comprises a layer of piezoelectric material, which will be referred to as piezo layer 12 below, and is located between a first electrode, or top electrode T, and a second electrode, or bottom electrode B. The designations top electrode and bottom electrode are just for explanation purposes and do not represent any limitation with regard to the spatial arrangement and positioning of the acoustic resonator.
If an electric field is applied between first electrode T and second electrode B of acoustic resonator 10, the reciprocal or inverse piezoelectric effect will cause acoustic resonator 10 to mechanically expand or contract, the case of expansion or of contraction depending on the polarization of the piezoelectric material. This means that the opposite case applies if the electric field is inversely applied between the T and B electrodes. In the case of an alternating field, an acoustic wave is generated in piezo layer 12, and, depending on the implementation of acoustic resonator 10, this wave will propagate, for example, in parallel with the electric field, as a longitudinal wave, or, as a transversal wave, transverse to the electric field, and will be reflected, for example, at the acoustic interface of the resonator with the outside world. For longitudinal waves, whenever the thickness d of piezo layer 12 and of the top and bottom electrodes can accommodate an integer multiple of half the wavelength λ of the acoustic waves, resonance states and/or acoustic resonance vibrations will occur. Because each acoustic material has a different propagation velocity for the acoustic wave, the fundamental resonance frequency, i.e. the lowest resonance frequency FRES, will be inversely proportional to the weighted sum of all thicknesses of the resonator layers.
The piezoelectric properties and, thus, also the resonance properties of an acoustic resonator depend on various factors, e.g. on the piezoelectric material, the fabrication method, the polarization impressed upon the piezoelectric material during fabrication, and the quality of the piezo material (i.e. the size of the crystals). As has been mentioned above, the resonance frequency in particular depends on the total thickness of the resonator.
FIG. 2 shows a model of a bulk acoustic wave (BAW) device or thin film bulk acoustic resonator (FBAR). The model of FIG. 2 is a modified Butterworth-Van Dyke model (MBVD) model. For a high quality resonator, the resistance values Rs, Ro, and Rm are small, in which case they can be neglected at the frequencies of interest. In that case, for simplification, the device can be modeled by the series-resonant combination of Lm and Cm, in parallel with a capacitance Co. The frequency response of this model is a bandpass response, with frequencies below the passband being attenuated mainly by the capacitors Cm and Co, and with frequencies above the passband being attenuated mainly by the inductance Lm.
As noted above, acoustic resonators can be employed in electrical filters, and in particular in radio frequency (RF) and microwave filters. These resonators can be combined in various ways to produce a variety of filter configurations. One particular configuration is a coupled resonator filter (CRF) wherein a coupling layer combines the acoustic action of the two acoustic resonators, which leads to a bandpass filter transfer function. A typical device includes two acoustic stacks, each comprising a layer of piezoelectric material disposed between two electrodes. A decoupling material is disposed between the acoustic stacks. Acoustic waves achieve resonance across the acoustic stacks, with the resonant frequency of the waves being determined by the materials in the acoustic stack.
FIG. 3 shows a device 300 including two acoustically coupled acoustic resonators 310, 320 having an acoustic coupling layer 330 between them.
Device 300 may operate as a bandpass filter, receiving an input signal applied to the input terminal 305 connected to the first acoustic resonator 310, and providing a bandpass-filtered output signal at output terminal 355.
Device 300 may be employed, for example, as a bandpass filter in a cellular or mobile telephone.
In many applications, it is useful for a filter such as device 300 also to provide an electrical impedance transformation from an input having one electrical impedance to an output having another electrical impedance. For example, in many communication devices, an antenna is used to receive signal and to transmit signals. The received signals are provided to a receiving amplifier of a receiver of the communication device. Moreover, the antenna may receive signals from a transmitter amplifier of a transmitter. Regardless of whether the transmission/reception of signals is half or full duplex, or even simplex, often times the antenna has an impedance that varies from the impedance of the amplifier (receiver or transmitter). As should be appreciated, mismatched impedances result in reflections and losses that are beneficially avoided.
Coupled resonator filters with which provide an impedance transformation have been previously disclosed. For example, one such solution involves using different piezoelectric thicknesses for the two resonators and/or using electrodes with different metals. However, in practice these approaches can have drawbacks, including in some cases low effective coupling constant for one of the two resonators (with negative implications on the filter insertion loss), and manufacturing difficulty in using different materials and/or trying to tightly control the piezoelectric materials' thicknesses in order to control the impedance transformation ratio to be at a desired value.
What is needed, therefore, is an acoustic resonator filter structure that can overcome at least some of the shortcomings of known electrical impedance transformers discussed above.